1. Field of the Invention
The invention relates generally to the field of drilling boreholes through subsurface rock formations. More specifically, the invention relates to methods for determining borehole fluid control events, such as loss of drilling fluid or formation fluid entry into a borehole.
2. Background Art
The exploration for and production of hydrocarbons from subsurface Earth formations ultimately requires a method to reach and extract the hydrocarbons from the formations. The reaching and extracting are typically performed by drilling a borehole from the Earth's surface to the hydrocarbon-bearing Earth formations using a drilling rig. In its simplest form, a land-based drilling rig is used to support a drill bit mounted on the end of a drill string. The drill string is typically formed from lengths of drill pipe or similar tubular segments connected end to end. The drill string is supported by the drilling rig structure at the Earth's surface. A drilling fluid made up of a base fluid, typically water or oil, and various additives, is pumped down a central opening in the drill string. The fluid exits the drill string through openings called “jets” in the body of the rotating drill bit. The drilling fluid then circulates back up an annular space formed between the borehole wall and the drill string, carrying the cuttings from the drill bit so as to clean the borehole. The drilling fluid is also formulated such that the hydrostatic pressure applied by the drilling fluid is greater than surrounding formation fluid pressure, thereby preventing formation fluids from entering into the borehole.
The fact that the drilling fluid hydrostatic pressure typically exceeds the formation fluid pressure also results in the fluid entering into the formation pores, or “invading” the formation. To reduce the amount of drilling fluid lost through such invasion, some of the additives in the drilling fluid adhere to the borehole wall at permeable formations thus forming a relatively impermeable “mud cake” on the formation walls. This mud cake substantially stops continued invasion, which helps to preserve and protect the formation prior to the setting of protective pipe or casing in the borehole as part of the drilling process, as will be discussed further below. The formulation of the drilling fluid to exert hydrostatic pressure in excess of formation pressure is commonly referred to as “overbalanced drilling.”
The drilling fluid ultimately returns to the surface, where it is transferred into a mud treating system, generally including components such as a shaker table to remove solids from the drilling fluid, a degasser to remove dissolved gases from the drilling fluid, a storage tank or “mud pit” and a manual or automatic means for addition of various chemicals or additives to the fluid treated by the foregoing components. The clean, treated drilling fluid flow is typically measured to determine fluid losses to the formation as a result of the previously described fluid invasion. The returned solids and fluid (prior to treatment) may be studied to determine various Earth formation characteristics used in drilling operations. Once the fluid has been treated in the mud pit, it is then pumped out of the mud pit and is pumped into the top of the drill string again.
The overbalanced drilling technique described above is the most commonly used formation fluid pressure control method. Overbalanced drilling relies primarily on the hydrostatic pressure generated by the column of drilling fluid in the annular space (“annulus”) to restrain entry of formation fluids into the borehole. By exceeding the formation pore pressure, the annulus fluid pressure can help prevent sudden influx of formation fluid into the borehole, such as gas kicks. When such gas kicks occur, the density of the drilling fluid may be increased to prevent further formation fluid influx into the borehole. However, the addition of density increasing (“weighting”) additives to the drilling fluid: (a) may not be rapid enough to deal with the formation fluid influx; and (b) may cause the hydrostatic pressure in the annulus to exceed the formation fracture pressure, resulting in the creation of fissures or fractures in the formation. Creation of fractures or fissures in the formation typically results in drilling fluid loss to the formation, possibly adversely affecting near-borehole permeability of hydrocarbon-bearing formations. In the event of gas kicks, the borehole operator may elect to close annular sealing devices called “blow out preventers” (BOPs) located below the drilling rig floor to control the movement of the gas up the annulus. In controlling influx of a gas kick, after the BOPs are closed, the gas is bled off from the annulus and the drilling fluid density is increased prior to resuming drilling operations.
The use of overbalanced drilling also affects the depths at which casing must be set during drilling operations. The drilling process starts with a “conductor pipe” being driven into the ground. A BOP stack is typically attached to the top of the conductor pipe, and the drilling rig positioned above the BOP stack. A drill string with a drill bit may be selectively rotated by rotating the entire string using the rig kelly or a top drive, or the drill bit may be rotated independent of the drill string using a drilling fluid powered motor installed in the drill string above the drill bit. As noted above, an operator may drill through the Earth formations (“open hole”) until such time as the drilling fluid pressure at the drilling depth approaches the formation fracture pressure. At that time, it is common practice to insert and hang a casing string in the borehole from the surface down to the lowest drilled depth. A cementing shoe is placed on the drill string and specialized cement is displaced through the drill string and out the cementing shoe to travel up the annulus and displace any fluid then in the annulus. The cement between the formation wall and the outside of the casing effectively supports and isolates the formation from the well bore annulus. Further open hole drilling can be carried out below the casing string, with the drilling fluid again providing pressure control and formation protection in the drilled open hole below the bottom of the casing. The casing protects the shallower formations from fracturing induced by the hydrostatic pressure of the drilling fluid when the density of the fluid must be increased in order to control formation fluid pressures in deeper formations.
FIG. 1 is an exemplary diagram of the use of drilling fluid density to control formation pressures during the drilling process in an intermediate borehole section. The top horizontal bar represents the hydrostatic pressure exerted by the drilling fluid and the vertical bar represents the total vertical depth of the borehole. The formation fluid (pore) pressure graph is represented by line 10. As noted above, in overbalanced drilling, the drilling fluid density is selected such that its pressure exceeds the formation pore pressure by some amount for reasons of pressure control and borehole stability. Line 12 represents the formation fracture pressure. Borehole fluid pressures in excess of the formation fracture pressure can result in the drilling fluid pressurizing the formation walls to the extent that small cracks or fractures will open in the borehole wall. Further, the drilling fluid pressure overcomes the formation pressure and causes significant fluid invasion. Fluid invasion can result in, among other problems. reduced permeability, adversely affecting formation production. The pressure generated by the drilling fluid and its additives is represented by line 14 and is generally a linear function of the total vertical depth. The hydrostatic pressure that would be generated by the fluid absent any additives, that is by plain water, is represented by line 16.
In an “open loop” drilling fluid system described above, where the return fluid from the borehole is exposed only to atmospheric pressure, the annular pressure in the borehole is essentially a linear function of the borehole fluid density with respect to depth in the borehole. In the strictest sense this is true only when the drilling fluid is static. In reality the drilling fluid's effective density may be modified during drilling operations due to friction in the moving drilling fluid, however, the resulting annular pressure is generally linearly related to vertical depth.
In the example of FIG. 1, the hydrostatic pressure 16 of the drilling fluid and the pore pressure 10 generally track each other in the intermediate section of the borehole to a depth of approximately 7000 feet. Thereafter, the pore pressure 10 (pressure of fluids in the pore spaces of the Earth formations) increases at a rate above that of an equivalent column of water in the interval from a depth of 7000 feet to approximately 9300 feet. Such abnormal formation pressures may occur where the borehole penetrates a formation interval having significantly different characteristics than the prior formation. The hydrostatic pressure 14 maintained by the drilling fluid is safely above the pore pressure prior to about 7000 feet. In the 7000-9300 foot interval, the differential between the pore pressure 10 and hydrostatic pressure 14 is significantly reduced, decreasing the margin of safety during drilling operations. A gas kick in this interval may result if the pore pressure exceeds the hydrostatic pressure, with an influx of fluid and gas into the borehole possibly requiring activation of the BOPs. As noted above, while additional weighting material may be added to the drilling fluid to increase its hydrostatic pressure, such will be generally ineffective in dealing with a gas kick due to the time required to increase the fluid density at the kick depth in the borehole. Such time results from the fact that the drilling fluid must be moved through thousands of feet of drill pipe to even reach the bit depth, let alone begin filling the annulus to increase the hydrostatic pressure in the annulus.
To overcome the foregoing limitations of drilling using an open-loop fluid circulating system, there have been developed a number of drilling systems called “dynamic annular pressure control” (DAPC) systems. One such system is disclosed, for example, in U.S. Pat. No. 6,904,981 issued to van Riet and assigned to Shell Oil Company. The DAPC system disclosed in the '981 patent includes a fluid backpressure system in which fluid discharge from the borehole is selectively controlled to maintain a selected pressure at the bottom of the borehole, and fluid is pumped down the drilling fluid return system to maintain annulus pressure during times when the mud pumps are turned off. A pressure monitoring system is further provided to monitor detected borehole pressures, model expected borehole pressures for further drilling and to control the fluid backpressure system.
As may be inferred from the above discussion of fluid influx and fluid loss events, it is important that detection of such events, and corrective actions therefore take place as soon as possible after the beginning of any such event such that the corrective actions are most likely to be effective. This is particularly the case with gas kicks, because as a gas kick flows up the annulus, the hydrostatic pressure due to the intruding gas, is reduced, whereupon the gas increases in volume, thus displacing successively larger volumes of drilling fluid in the annulus. The displacement of drilling fluid results in reduction of hydrostatic pressure on the annulus, further exacerbating the gas expansion in a dangerous cycle. Much work has therefore been devoted to early, accurate detection of well control events. Many of the techniques known in the art for detection of well control events using open loop fluid circulation systems are described, for example, in U.S. Pat. No. 6,820,702 issued to Niedermayr et al. Generally, techniques known in the art for detecting well control events used with open loop fluid circulation systems use differences between fluid flow volume into the borehole and fluid flow out of the borehole to infer the presence of such an event. Further, well control event techniques known in the art rely on precision measurement of flow into and flow out of the wellbore for detection of the events.
What is needed are improved methods for determining existence of a well control events that may in some cases be used with a closed loop fluid circulation systems such as DAPC systems.